1. Field of the Invention
The present invention, relates to a Cathode-Ray Tube (CRT) system and more particularly, to a color or monochrome CRT system including a CRT and a controller subsystem for controlling the CRT, in which an electron gun of the CRT is able to generate a dynamic, electrostatic quadrupole lens.
2. Description of the Prior Art
With conventional CRT systems, generally, the deflecting magnetic-field of the self-convergence type is spatially non-uniform. This non-uniform magnetic field has a horizontal distribution like a pin-cushion shape and a vertical distribution like a barrel shape, and generates a "magnetic quadrupole lens".
An electron beam that has been emitted from an electron gun and that passes through the deflecting magnetic-field is applied with a horizontal diverging action or force and a vertical converging action or force from the magnetic quadrupole lens, and as a result, the beam is deformed. The electron beam thus deformed generates a beam spot with some defocus on the phosphor screen of the CRT.
The defocus of the electron-beam spot becomes conspicuous in the periphery of the phosphor screen compared with the central area thereof, because the strength or intensity of the magnetic quadrupole lens varies dependent upon the deflection level of the electron beam (i.e., the relative position of the beam spot on the screen with respect to the center of the screen).
Accordingly, there arises a problem that the resolution of the CRT system degrades in the periphery of the phosphor screen.
To solve the problem of the resolution degradation, an electron gun with a "dynamic, electrostatic quadrupole lens" has been developed and practically used.
FIG. 1 schematically shows a part of a conventional color CRT system using this electron gun, which is disclosed in the Japanese Non-Examined Patent Publication No. 1-232643 published in September 1989. This system has an electron gun emitting three electron beams for red (R), green (G), and blue (B). However, only one of the electron beams is explained here for the sake of simplification of description.
As shown in FIG. 1, an electron gun 111 has a cathode 112, a controlling electrode 113, a first accelerating electrode 114, a first focusing electrode 115, a second focusing electrode 116, and a second accelerating electrode 118, which are fixed on a pair of supports (not shown) made of an electrically insulating material such as glass. The cathode 112, the controlling electrode 113, the first and second accelerating electrodes 114 and 118, and the first and second focusing electrodes 115 and 116 are aligned at specific intervals along the central axis of the electron gun 111.
The cathode 112 emits an electron beam (not shown). The controlling electrode 113 controls the amount of the electron beam emitted from the cathode 112. The first and second accelerating electrodes 114 and 118 accelerate the electron beam. The first and second focusing electrodes 115 and 116 focus the electron beam on a phosphor screen (not shown) of the conventional CRT system.
The conventional color CRT system of FIG. 1 further has a power supply 119 for supplying a first focusing voltage E.sub.1 to the first focusing electrode 115 and a second focusing voltage E.sub.2 to the second focusing electrode 116. The first focusing voltage E.sub.1 is supplied to the first focusing electrode 115 through a resistor 120 with a resistance R. The second focusing voltage E.sub.2 is supplied directly to the second focusing electrode 116.
As shown in FIG. 2, the first focusing voltage E.sub.1 is a dc voltage, which is obtained by division of the second focusing voltage E.sub.2 by the resistor 120. The second focusing voltage E.sub.2 is an ac voltage with a parabolic waveform, which is generated by superposing an ac component with a parabolic waveform on a dc component in the power supply 119. The second focusing voltage E.sub.2 is synchronized with the deflecting ac current. A voltage difference exists between the first and second focusing electrodes 115 and 116.
With the conventional color CRT system of FIG. 1, due to the application of the first and second focusing voltages E.sub.1 and E.sub.2, an electrostatic quadrupole lens (not shown) is generated in a space 117 between the opposing surfaces of the first and second focusing electrodes 115 and 116. The electron beam emitted from the cathode gun 112 is applied with a horizontal converging action or force and a vertical diverging action or force from the electrostatic quadrupole lens thus generated.
Consequently, the above beam-spot deformation action due to the magnetic quadrupole lens is compensated by the beam-spot deformation action due to the electrostatic quadrupole lens thus generated. As a result, the deformation or defocus of the beam-spot is substantially deleted within the whole phosphor screen.
Further, with the conventional color CRT system of FIG. 1, the first focusing voltage E.sub.1 is obtained by dividing the second focusing voltage E.sub.2 with the use of the resistor 120. Because of the action of the resistor 120, the parabolic ac component of the second focusing voltage E.sub.2 is substantially removed, resulting in the first focusing voltage E.sub.1 containing only the dc component. The value of the first focusing voltage E.sub.1 will be equal to a time average of the second focusing voltage E.sub.2.
This configuration of the single power supply as shown in FIG. 1 leads to an advantage that only one power supply is needed for supplying the first and second focusing voltages E.sub.1 and E.sub.2. However, on the other hand, this configuration means that the first focusing voltage E.sub.1 is automatically determined by the second focusing voltage E.sub.2. In other words, the difference between the first and second focusing voltages E.sub.1 and E.sub.2 cannot be adjusted by changing the amplitude of the second focusing voltage.
This automatic determination of the first focusing voltage E.sub.1 will cause a problem that the focusing characteristic or performance of the CRT system is unable to be adjusted in response to the characteristic or performance variation and/or deviation of the actual electron gun 111. In other words, the operation of the electron gun 111 is not always optimized, resulting in resolution degradation.